Conventionally, as a source circuit including a particular type of protecting circuit, a circuit may include: a first comparator configured to output a first output signal, a second comparator configured to output a second output signal, and a determining unit configured to determine the presence or absence of conduction angle control of an AC voltage and whether the conduction angle control is a phase control system or an opposite phase control system based on a time difference between the first output signal and the second output signal.
On the other hand, there is a trade-off relationship of discharge capacity and response speed with power consumption for a shunt circuit that includes a comparator and a transistor to discharge electric current based on an input voltage of a predetermined value in order to protect a protected circuit. Therefore, in such a shunt circuit, it is difficult to discharge electric current sufficient for a sharply varying input voltage while suppressing power consumption.